This invention relates to a light source. In particular, the invention relates to a light source in the form of a light emitting diode (LED) package suitable for use in an LED matrix video display.
Light emitting diodes (LEDs) fabricated from silicon wafer are commonly used to generate light in a variety of applications ranging from simple low-power indication lights to higher-power LED traffic light clusters and LED matrix video displays. Typically, the light emitting diode die is assembled into a sealed package containing electrical connections between the die and terminal pads exposed on an outer surface of the package. Such a package enables simple connection of the diode to external circuitry and, due the sealing properties of the package, protects the die from external damage.
Recently, there has been a drive to make smaller surface mount LED packages which allow the LED to be reliably mounted onto a printed circuit board substrate at relatively high speeds. By making individual LED packages smaller, the number of LED dies per unit area in a multiple LED package may be increased. Furthermore, when the LED is mounted onto a circuit board, the thickness of the assembled circuit board can be reduced.
One application of small surface mount LED packages is in the field of large-scale video displays or video walls that are often found on the outside of large buildings or at large sporting arenas. The level of luminosity of such a video display needs to be as high as possible in order for the display to have good contrast and to appear highly visible to spectators. In view of this need, the increased LED area density provided by surface mount LED packages is significant as it can lead to more light being generated per unit area of the display. Furthermore, the use of surface mount LED packages to increase the LED area density of the display also improves the maximum pixel resolution that may be achieved by the display.
Today""s surface mount LED packages are available in a wide variety of configurations. FIG. 1 shows one typical surface mount LED package 100 comprising an LED die 110 mounted on a circuit board substrate 120 with a transparent material 130 encapsulating the LED 110. The package includes a pair of conductive interconnects 140, 142 for coupling the LED to external circuitry. A first electrode on the bottom surface of the LED 110 is mounted on and electrically coupled to one of the pair of conductive interconnects 140. A very small wire 144 is then xe2x80x9cbondedxe2x80x9d or welded at one end to a second electrode on the top surface of the LED 110, and at the other end to the other one of the pair of conductive interconnects 142.
A characteristic of the package of FIG. 1 is that light generated by the LED has a relatively wide directivity. Accordingly, when this type of LED package is incorporated into a video display, the video display will have a relatively wide angle of view in both horizontal and vertical directions. However, a problem with the wide directivity of the LED package is that the light energy emitted by the LED is distributed over a larger angle making the LED appear dimmer in the forward viewing direction. Therefore, video displays incorporating the LED package of FIG. 1 will accordingly appear dimmer to spectators. Video displays are known to require only a viewing angle of around 60 degrees in the horizontal plane and around 30 degrees, in the vertical plane.
In order to overcome the drawbacks of using known surface mount LEDs in video matrix displays, U.S. Pat. No. 5,835,269 assigned to Sony Corporation, Japan, proposes using an video display apparatus with a unitary reflecting member in the form of a corrugated sheet. The corrugated sheet compensates for the wide directivity of known surface mount LEDs by reflecting stray light back into the forward viewing direction. However, the use of additional structure in the video display adds cost and complexity to the manufacture of video displays which is undesirable.
What is needed is a light emitting diode package which is optimised for use in video displays such that the light energy emitted by the light source may be concentrated in the forward viewing direction.
According to a first aspect of the present invention, there is provided a light emitting diode package comprising: a planar substrate having an upper surface and a lower surface, a portion of the upper surface defining a recess, said recess having a side wall tapering outwards towards the upper surface; a light emitting diode mounted in the recess of the substrate; a first electrically-conductive interconnect extending between the upper and lower surfaces, the first interconnect having a terminal on the upper surface coupled to the light emitting diode and an exposed pad on the lower surface for coupling to external circuitry; a second electrically-conductive interconnect extending between the upper and lower surfaces, the second interconnect having a terminal on the upper surface coupled to the light emitting diode and a conductive pad on the lower surface for coupling to external circuitry; and a transparent encapsulant material bonded to the first surface of the substrate to encapsulate the light emitting diode, the encapsulant material being molded to form a focussing ellipsoidal dome over the light emitting diode.
A light emitting diode package in accordance with the invention has the advantage that light energy emitted by the light emitting diode is concentrated by a combination of reflection from the walls of the substrate recess and by refraction from the focussing ellipsoidal dome. The light emitting diode package also provides the additional advantage that by mounting the light emitting diode in the recess of the substrate the package occupies a smaller volume.
Preferably, the focussing ellipsoidal dome is shaped to concentrate light within a range of angles in the horizontal and vertical planes. The light emitting diode package is thus optimised for use in LED matrix video displays that require only certain viewing angles in the horizontal and vertical planes.
In a preferred embodiment, the side wall of the recess is plated with a metallic layer presenting a silvered reflective surface to the light emitting diode. The silvered reflective surface provides improved reflectivity over previously employed gold layers which present a golden appearance to the light emitting diode.
Suitably, the metallic layer forms the terminal of the first interconnect.
Ideally, the substrate defines first and second vias extending between the upper and lower surfaces, a portion of each of the first and second interconnects extending through the first and second vias respectively.
Each of the conductive pads of the first and second interconnects may include a gold plated layer for electrically coupling to external circuitry.